This invention relates to a method of manufacturing a polymer light guide. In particular, it relates to a method that produces a light guide that is essentially free from voids and gas bubbles.
Production of light guides from polymeric materials is well known. A variety of different techniques for their production may be found in the literature.
Reference may be had to U.S. Pat. No. 4,422,719 in the name of Orcutt. Orcutt describes a flexible thermosetting polymer produced as a light guide. The polymer composition is disclosed as 60-90% allyl diglycol carbonate (CR39), a 1.5 to 3% solution of isopropylpercarbonate (IPP) and the balance methyl methacrylate. The monomer solution is polymerised in a hot liquid bath for a period of 12 to 24 hours. Orcutt discloses that the core material may be under catalysed in order for it to remain flexible.
Reference may also be had to U.S. Pat. No. 5,298,327 in the name of Zarian. Zarian describes a plastic light conduit of cross-link polymer material having good light transmitting characteristics. Also disclosed is a method of manufacturing the light conduit involving progressive heating of reactive monomer mixture in a tube which is submerged in a progressive reactor utilizing cold oil to maintain a cold, non-reacting zone, and hot water to create a hot zone that causes reaction, while simultaneously carrying away the excess exothermic heat of reaction.
Although the Zarian method may produce polymer light guides of good quality that are free from bubbles and voids it is a slow technique. Furthermore, the progressive nature of the method places a practical limit on the length of light guide that can be produced due to difficulties in handling tubes filled with unreacted liquid monomer.
It is an object of the present invention to provide a method of manufacturing a polymer light guide that substantially alleviates the deficiencies in the prior known methods.
In one form, although it need not be the only or indeed the broadest form,.the invention resides in a method of manufacturing a polymer light guide including the steps of:
producing a monomeric mixture from amounts of monomer, initiator and cross-linking agent;
forming a polymer tube into a coil;
filling the coil of polymer tube with the monomeric mixture;
pressurising and heating the full length of the polymer tube to conditions appropriate to initiate and maintain polymerisation of the mixture.
In preference, the method further includes the step of stirring the monomeric mixture to form a uniform mixture. The mixture is preferably allowed to degas for a period of time. The period of time is typically in the order of 30 minutes.
The method may further include the step of purification of the monomer before producing the monomeric mixture. Purification may be by vacuum distillation, ion exchange filtration or other suitable process.
The method may also include the step of pre-expanding the polymer tube by pressurising the tube prior to filling the tube with the monomeric mixture.
The polymer tube is preferably greater than 100 metres in length. More preferably the polymer tube is greater than 300 metres in length. In addition it is preferable that the coil of polymer tubing is formed by winding polymer tube onto a drum.
The step of pressurising and heating the full length of the polymer tube may suitably be conducted in two or more steps in which the mixture in the polymer tube is subjected to a first set of temperature and pressure conditions followed by second and further sets of temperature and pressure.
The conditions appropriate for initialisation of polymerisation include a pressure in the range 10 kpa to 2000 kpa.
The temperature is preferably maintained in a range between the storage temperature of the mixture and a temperature whereby the half life of the initiator used is between 10 and 20 hours. The temperature of the mixture is preferably increased to a value whereby the half life of the initiator used, is between 5 and 10 hours. The temperature may be further increased to a value whereby the half life of the initiator used is between 30 minutes and 60 minutes to obtain a desired final cure.
In a further form the invention resides in an apparatus suitable for manufacturing a polymer light guide, said apparatus comprising,
a means for supporting a coil of polymer tube;
a reaction vessel for holding a polymer tube fillable with a monomeric mixture;
pressure means connectable to the polymer tube for applying pressure to the tube before and after filling said tube with said monomeric mixture;
a temperature controlled fluid, circulable through the reaction vessel so as to control the temperature within the reaction vessel;
a pump for circulating the temperature controlled fluid; and
temperature control means for adjusting the temperature of the temperature controlled fluid.
In preference the polymer tube is completely immersed in the fluid within the reaction vessel. The reaction vessel may be a room. The means of supporting the coil of polymer tube may be a drum.
The fluid may conveniently be water although other fluids, such as oil, will be necessary if high temperature initiators are used. Air may also be used as the fluid for temperature control. In the case of air, the pump is conveniently a fan. Air will be the fluid of choice when the reaction vessel is a room.
The apparatus may further include a filling means. The filling means suitably comprises a pressure vessel containing a reservoir of monomeric mixture, a bleeding tank for receiving excess monomeric mixture and pressure supply means for supplying pressure to the pressure vessel to force monomerix mixture through the polymer tube when connected between the pressure vessel and the bleeding tank.
Polishing means may also be incorporated into the apparatus for polishing the optical entrance and exit faces of the polymer light guides produced in the apparatus.